a) Field of the Invention
The present invention relates to a variable gain amplifier circuit, and more particularly to a variable gain amplifier circuit suitable for use with a CMOS integrated circuit which processes an image signal.
b) Description of the Related Art
A variable gain amplifier circuit such as shown in FIG. 11 has been proposed as cited in JP-B-2-34205 as a prior art. This variable amplifier uses an operational amplifier 100 as an inverter amplifier and can change an amplifier gain by changing a resistance ratio of an input resistor 101 to a feedback resistor 102.
The input resistor 101 is made of a ladder resistor network of an eight-stage structure having resistors R and 2R. A gain is determined by a combination of turned-on switches. Namely, an input voltage Vin is amplified by the operational amplifier 100 and output as an output voltage Vout at a gain determined by a combination of switches 103 whose contacts are connected to the inverting input terminal of the operational amplifier 100 (on-state switches 103).
In the variable gain amplifier circuit shown in FIG. 11 according to a related art, the operational amplifier 100 is required to have a very high gain-bandwidth product (GB product) if both the gain and bandwidth necessary for image signal processing are to be satisfied. It is therefore very difficult to practically realize a variable gain amplifier circuit for image signal processing by using CMOS integrated circuit techniques.
As apparent from the circuit shown in FIG. 11, if the input voltage Vin is replaced by a reference voltage Vref, this variable gain amplifier circuit is quite the same as a D/A converter which converts a digital signal constituted of bits S1 to S8 into an analog signal Vout.